Yours is a classic "cheating" dream, which is often misunderstood. There are many excellent examples of and explanations for cheating dreams on this site, especially in the romance dreams section:
dreamdoctor.com/common-dreams-their-mean.../dreams-romance.html
First, here's a link to its definition in the dream dictionary:
dreamdoctor.com/dream-dictionary/definitions/cheating.html There's also a related explanation in the "Listen Now!" section of the homepage titled: "meaning of: ex-lover (video)"
Your "cheating" is not about a desire for that person, but for certain qualities that that person has, qualities that you miss or want in your fiance and/or relationship. Interestingly, the person you are cheating with is a former teacher. You "lust for" for what he represents to you. Could it be his authority, knowledge, guidance and/or his tutelage in the classroom of love?
Once you identify what you are missing or needing in your relationship with your fiance (personified by your former teacher), find a way to talk to him about it. Odds are he will want to know about it and even work on it. Better to address it now than to go wanting (or "cheating") during your marriage.
Keep paying attention to your dreams. They will always let you know what's right or wrong about your life. The rest is up to you.
Hope that helps!